Raw natural gas comprises primarily methane and also contains numerous minor constituents which may include water, hydrogen sulfide, carbon dioxide, mercury, nitrogen, and light hydrocarbons typically having two to six carbon atoms. Some of these constituents, such as water, hydrogen sulfide, carbon dioxide, and mercury, are contaminants which are harmful to downstream steps such as natural gas processing or the production of liquefied natural gas (LNG), and these contaminants must be removed upstream of these processing steps. The hydrocarbons heavier than methane typically are condensed and recovered as natural gas liquids (NGL) and fractionated to yield valuable hydrocarbon products.
NGL recovery utilizes cooling, partial condensation, and fractionation steps that require significant amounts of refrigeration. This refrigeration may be provided by work expansion of pressurized natural gas feed and vaporization of the resulting condensed hydrocarbons. Alternatively or additionally, refrigeration may be provided by external closed-loop refrigeration using a refrigerant such as propane. It is desirable to recover NGL from pressurized natural gas without reducing the natural gas pressure significantly. This allows the natural gas product (for example, pipeline gas or LNG) to be provided at or slightly below the feed pressure so that feed and/or product recompression is not required.
In order to recover NGL and natural gas products at near feed pressure while minimizing refrigeration power consumption, improved NGL recovery processes are needed. The present invention, which is described below and defined by the claims that follow, provides an improved lean oil absorption-type NGL recovery process which can be operated at pressures significantly above the critical pressure of methane, wherein the natural gas feed pressure need not be reduced in the process.
Embodiments of the invention include a process for the recovery of components heavier than methane from natural gas, wherein the process comprises
(a) cooling a natural gas feed to provide a cooled natural gas feed and introducing the cooled natural gas feed into an absorber column at a first location therein;
(b) withdrawing from the absorber column a first overhead vapor stream depleted in components heavier than methane and a bottoms stream enriched in components heavier than methane;
(c) introducing a methane-rich reflux stream at a second location in the absorber column above the first location;
(d) separating the bottoms stream into a stream enriched in methane and one or more streams enriched in components heavier than ethane; and
(e) introducing an absorber liquid comprising components heavier than ethane into the absorber column at a location between the first location and the second location.
The process may further comprise combining all or a portion of any of the one or more streams enriched in components heavier than ethane in (d) with the methane-rich reflux stream in (c). Alternatively, the process may further comprise withdrawing all or a portion of any of the one or more streams enriched in components heavier than ethane in (d) as a product stream. The natural gas feed may be at a pressure above 600 psia.
The absorber liquid may comprise components obtained from any of the one or more streams enriched in components heavier than ethane in (d). The absorber liquid may contain greater than 50 mole % of hydrocarbons containing five or more carbon atoms. Alternatively, the absorber liquid may contain greater than 50 mole % of hydrocarbons containing four or more carbon atoms. In another alternative, the absorber liquid may contain greater than 50 mole % of hydrocarbons containing three or more carbon atoms.
The absorber liquid may be cooled by indirect heat exchange with a vaporizing recirculating refrigerant prior to being introduced into the absorber column. This vaporizing recirculating refrigerant may be propane.
The process may further comprise cooling and partially condensing the first overhead vapor stream to form a two-phase stream, separating the two-phase stream to provide a second overhead vapor stream and the methane-rich reflux stream in (c). The second overhead vapor stream may be recovered as a product stream depleted in components heavier than methane. All or a portion of any of the one or more streams enriched in methane in (d) may be combined with the first overhead vapor stream prior to separating the two-phase stream.
The refrigeration for cooling and partially condensing the first overhead vapor stream may be provided by indirect heat exchange with a vaporizing refrigerant. This vaporizing refrigerant may be a multi-component refrigerant.
The process may further comprise cooling, condensing, and subcooling the second overhead vapor stream to provide a liquefied natural gas product. All or a portion of the refrigeration required to cool, condense, and subcool the second overhead vapor stream may be provided by indirect heat exchange with a vaporizing refrigerant. This vaporizing refrigerant may be a multi-component refrigerant.
All or a portion of the refrigeration required to cool, condense, and subcool the second overhead vapor stream may be provided by indirect heat exchange with a cold refrigerant provided by work expansion of a compressed refrigerant comprising nitrogen.
All or a portion of the cooling of the natural gas feed may be provided by indirect heat exchange with one or more streams of vaporizing refrigerant. This vaporizing refrigerant may be propane.
The process may further comprise providing a portion of the cooling of the natural gas feed by indirect heat exchange with a liquid bottoms stream from the absorber column, thereby providing a vaporized bottoms stream, and introducing the vaporized bottoms stream into the absorber column to provide boilup vapor.
The process may further comprise cooling, condensing, and subcooling the stream enriched in methane in (d) to provide a liquefied methane-rich product. All or a portion of the refrigeration required to cool, condense, and subcool the stream enriched in methane may be provided by indirect heat exchange with the vaporizing refrigerant. Alternatively, all or a portion of the refrigeration required to cool, condense, and subcool the stream enriched in methane may be provided by indirect heat exchange with a cold refrigerant provided by work expansion of a compressed refrigerant comprising nitrogen. The liquefied methane-rich product may be combined with the liquefied natural gas product.
Embodiments of the invention also include a system for recovery of components heavier than methane from natural gas, wherein the system comprises
(a) an absorber column for separating natural gas into a methane-rich stream and a stream enriched in components heavier than methane;
(b) cooling means to cool a natural gas feed to provide a cooled natural gas feed and means for introducing the cooled natural gas feed into the absorber column at a first location therein;
(c) means for withdrawing from the absorber column a first overhead vapor stream depleted in components heavier than methane and a bottoms stream enriched in components heavier than methane;
(c) means for introducing a methane-rich reflux stream at a second location in the absorber column above the first location;
(d) separation means for separating the bottoms stream into a stream enriched in methane and one or more streams enriched in components heavier than ethane; and
(e) means for introducing an absorber liquid comprising components heavier than ethane into the absorber column at a location between the first location and the second location.
The system may further comprise means for cooling and partially condensing the first overhead vapor stream to form a two-phase stream and means for separating the two-phase stream to provide a second-overhead vapor stream and the methane-rich reflux stream. The system may further comprise a main heat exchanger having flow passages therein for cooling and partially condensing the first overhead vapor stream by indirect heat exchange with a vaporizing multi-component refrigerant, having flow passages therein for cooling a compressed multi-component refrigerant, pressure reduction means for reducing the pressure of the multi-component refrigerant to yield the vaporizing multi-component refrigerant, and means for distributing the vaporizing multi-component refrigerant in the main heat exchanger.
The system may further comprise additional flow passages in the main heat exchanger for cooling and at least partially condensing the second overhead vapor stream to provide a liquefied natural gas product. In addition, the system may further comprise a product heat exchanger wherein the liquefied natural gas product is further cooled by indirect heat exchange with a cold refrigerant provided by work expansion of a compressed refrigerant comprising nitrogen.